In semiconductor manufacturing, photolithography is used in the formation of integrated circuits on a semiconductor wafer. During a lithographic process, a form of radiant energy such as ultraviolet light is passed through a mask/reticle and onto the semiconductor wafer. The mask contains light restricting regions (for example totally opaque) and light transmissive regions (for example totally transparent) formed in a predetermined pattern. A grating pattern, for example, may be used to define parallel-spaced conductive lines on a semiconductor wafer. The wafer is provided with a layer of photosensitive resist material, commonly referred to as photoresist. Ultraviolet light passed through the mask onto the layer of photoresist thereby transfers the mask pattern therein. The resist is then developed to remove either the exposed portions of resist for a positive resist or the unexposed portions of the resist for a negative resist. The remaining patterned resist can then be used as a mask on the wafer during a subsequent semiconductor fabrication step, such as ion implantation or etching relative to layers on the wafer beneath the resist.
As microcircuit densities have increased, the size of the features of semiconductor devices has decreased to the submicron level. These submicron features may include the width and spacing of metal conducting lines or the size of various geometric features of active semiconductor devices. Utilization of submicron features in semiconductor manufacture has resulted in the development of improved lithographic processes and systems. One such improvement is known as phase shift lithography. With this technique, the interference of light rays is used to overcome diffraction and improve the resolution and depth of optic images projected onto a target. In phase shift lithography, the phase of an exposure light at the object is attempted to be controlled such that adjacent bright areas are formed preferably 180.degree. out-of-phase with one another. Better dark regions are thus produced between the bright areas by destructive interference even when diffraction would otherwise cause these areas to be less defined at the edges or otherwise lit to some degree. This technique improves total resolution at the object and has been used where resolutions fall to well below one micron, such as at 0.25 micron and below.
Non-phase shift lithographic masks generally contain only transparent and opaque areas. A phase shifting mask is constructed with the repetitive pattern formed of three distinct regions. A first region or layer comprises light restricting material that ideally allows no light transmission. A second light transmissive layer or region provides areas which preferably allow near 100% of incident light to pass therethrough. A third phase shifting area also preferably allows near 100% of the light to pass through, but at a preferred phase shifted 180.degree. from the light passing through the second non-phase shifted light transmissive area. The light transmissive areas and phase shift areas are situated such that light rays diffracted from the edges of the light restricting material and through the light transmissive and phase shift areas are essentially canceled out in a darkened area therebetween. This creates a pattern of dark and bright areas which can be used to clearly delineate features of a pattern defined by the mask at submicron dimensions, and onto a photo-patterned semiconductor wafer.
Different techniques have been developed in the art for fabricating different types of phase shifting masks. One type of mask is formed on a transparent substrate, such as polished quartz. An opaque layer, formed of a material such as chromium, is deposited on the transparent substrate and etched with a pattern of apertures. This forms opaque areas on the mask which, combined with the pattern of apertures, carry the desired pattern. With a phase shifting mask, the transparent areas in phase shift areas are formed within the apertures in an alternating pattern with respect to the opaque areas.
The phase shift areas of the mask pattern may be formed by providing a phase shifting material into every other aperture (i.e., an additive process). Alternately, phase shift areas may be formed by etching a depression/groove in every other aperture (i.e., a subtractive process). With this type of phase shifting structure, the light passing through a grooved aperture travels a shorter distance within the substrate relative to light passing through an aperture formed over the full thickness of the substrate. Light beams exiting adjacent apertures of the mask therefor have a phase difference. This phase difference is preferably 180.degree., or some odd multiple thereof, so that light waves cancel out at the wafer.
The invention was principally motivated in connection with problems associated with subtractive phase shifting reticle processes, and utilizing highly reflective opaque material. Yet as will be appreciated by the artisan, the invention will have applicability in additive or other processes, and using other than reflective materials, including processes and materials yet to be developed, with the invention only being limited by the accompanying claims both as literally worded and as interpreted in accordance with the Doctrine of Equivalents.
An earlier of my inventions is described in U.S. Pat. No. 5,225,035, which issued on Jul. 6, 1993. That invention was motivated to overcome a problem created with different light diffraction/transmissive properties which occur when light exits etched and unetched portions of a reticle. The invention the subject of this disclosure was motivated in part as a further improvement over my U.S. Pat. No. 5,225,035 described invention as particularly motivated in use of reticles utilizing highly reflective, opaque materials having antireflective material formed thereover.
Specifically, chromium and other reflective metals are commonly used as the opaque regions in reticle fabrication. Such material is highly light reflective, which can further cause problems with respect to the pattern being produced in the photoresist on the wafer. One technique used to minimize the reflection is to provide an antireflective layer over the chromium layer prior to fabrication of the pattern on the reticle. One example class of antireflective materials is chromium oxides, either deposited onto the reticle or formed by oxidizing the outer portion of a deposited chromium layer. Unfortunately, the preferred embodiment etching of all the transparent areas and phase shift areas in the '035 disclosure also resulted in etching of at least some of the antireflective coating material from some of the opaque regions.